In dual file systems according to the prior art, each having one or more pairs of dual files, the following processing steps are taken to establish correspondence between media constituting dual files and units into which the media are assembled.
(1) When actuating a dual file system, the operating system of the dual file system automatically recognizes the names of the media assembled into volumes in an on-line state and, if it succeeds in recognizing two media to constitute a pair of dual files, will set, in allocation information, information to the effect that the volumes into which those media are assembled are to be allocated to the dual files. In accessing those dual files, those volumes are allocated to the dual files. Thereafter, the dual files for which the information is set in the allocation information are operated in a dual state, while other dual files are operated in a non-dual state.
(2) When operating a dual file system, if failure occurs to one of the media of a pair of dual files operated in a dual state, the operating system of the dual file system will place off-line state (blockade) the volume into which the blockaded medium is assembled, and performs control so as to operate the dual files on a single line. However, since information indicating the allocation of the volume is maintained in the allocation information, the dual state of the dual files is retained.
(3) If, after the occurrence of a failure as described in (2), the dual file system stops operating for any reason and is to be re-actuated subsequently, the volume in which the blockaded medium is assembled will remain in an off-line state, and the allocation information will have also been deleted as a result of the system stop, the dual files involving the blockaded medium cannot be operated in a dual state by using that volume. In other words, the dual files have to be operated in a non-dual state only by another medium than the blockaded one. This holds true irrespective of whether or not the recovery of the data in the blockaded medium has been completed by the time the dual files are to be re-operated. For a technique regarding data recovery in media, reference may be made to "ACOS-4/XVP DFCF/TBCF REFERENCE MANUAL, DFB81E-2, NEC Corporation, 1991, 1992" (DFCF: Dual File Control Facility, TBCF: Track Blockade Control Facility).
Thus, according to the prior art, information indicating the state of correspondence between the volumes and media regarding dual files is not inherited from before to after a system stop.
If the blockaded medium after the recovery of data is assembled into another volume in an on-line state and reactuated, the dual files including that blockaded medium can be operated in a dual state. However, from the viewpoint of the overall operation of a dual file system, the use of another volume should be avoided wherever practicable, and therefore such a manner of operation is not desirable.
According to the prior art described above, when a dual file system is to be re-actuated, information indicating the state of correspondence between the volumes and media regarding dual files before a system stop is not inherited by the re-actuated system. As a result, since no allocation of the volume in an off-line state into which the blockaded medium was assembled takes place for the dual files including the blockaded medium, there is the disadvantage that, after re-actuation, the dual files including the blockaded medium are operated in a non-dual state, and it is impossible to recover the dual file system into the same state of the volumes and media including the dual files as during the operation before the system stop.
An object of the present invention is to obviate this disadvantage of the prior art, and to provide an apparatus to manage information indicating correspondence between volumes and media in a dual file system which enables the dual file system after re-actuation to operate in the same state of the volumes and media as during the operation before the system stop.